But Flynn was not asked to step down until February 13, nearly a month into his tenure as the country's senior-most national security official.

That timeline has become a focus for Robert Mueller, the special counsel who is examining why it took 18 days for the White House to remove Flynn after a stark warning from Sally Yates, the acting attorney general.

She had told the White House counsel, Don McGahn, that Flynn most likely lied to Vice President Mike Pence about his calls with Kislyak.

That was far from the first time, however, that an official had urged Trump and his advisers to rethink bringing Flynn into the White House.

By January 26, Yates was at least the fourth person to sound the alarm. And she warned McGahn about Flynn in two meetings and a phone call before she was fired on January 30 for unrelated reasons.

Obama's warnings

Donald Trump and Barack Obama. Pablo Martinez Monsivais/AP The first warning came from President Barack Obama on November 10.

A former Obama White House official confirmed to Business Insider on Monday that Obama cautioned Trump against appointing Flynn "based on the president's experience with Flynn in his administration."

"President Obama underscored with President-elect Trump how important a role the NSA is — and how it demanded a serious person with sound judgment, impeccable credentials, and unimpeachable character," the official said, referring to the National Security Agency. "In other words, [Obama] relied heavily on his national security advisers and wanted to convey how critical the job had been in our administration."

After Flynn pleaded guilty earlier this month to lying to the FBI about his conversations with Kislyak, Ty Cobb, the White House special counsel, characterized Flynn as a "former Obama administration official." Trump has pointed to Flynn's security clearance renewal in January 2016 as evidence that high-level officials once trusted Flynn.

But Obama fired Flynn as the head of the Defense Intelligence Agency in 2014 — and Flynn's security clearance would have been renewed at the DIA level rather than by the upper echelons of the White House.

Susan Rice, a former national security adviser, said in April that the renewal of Flynn's security clearance by the DIA would have been "a very separate thing ... from the vetting that goes into the appointment of any senior White House official, or any senior administration official."

The Trump White House, by contrast, in its vetting of Flynn either failed to detect or overlooked the fact that Flynn had been lobbying on behalf of Turkish government interests throughout the latter half of 2016.

A top congressman's warnings

Rep. Elijah Cummings. Joshua Roberts/Reuters

Sean Spicer, the former Trump White House press secretary, said in March, just after Flynn registered as a foreign agent with the Justice Department, that Trump had not been aware that Flynn had been paid to lobby for Turkish interests in the months before the US election.

But The Daily Caller and Politico reported in November last year that Flynn's consulting firm, Flynn Intel Group, had been hired to do that.

And days later, on November 18 — just over a week after Obama warned Trump about Flynn — Rep. Elijah Cummings sent Pence a letter requesting more information about potential conflicts of interest posed by Flynn's lobbying work.

"Recent news reports have revealed that Lt. Gen. Flynn was receiving classified briefings during the presidential campaign while his consulting firm, Flynn Intel Group, Inc., was being paid to lobby the U.S. Government on behalf of a foreign government's interests," said Cummings, the ranking Democratic member of the House Committee on Oversight and Government Reform.

Cummings told Business Insider through his office at the time that he thought "the problems that have occurred with Lt. Gen. Flynn" could have been avoided had Pence heeded his warnings.

Pence headed Trump's transition team, but he has insisted that he did not know about Flynn's lobbying work or conversations with the Russian ambassador before Flynn's ouster in February.

Chris Christie's warnings

Gov. Chris Christie of New Jersey. Thomson Reuters Gov. Chris Christie of New Jersey recently suggested he thought he was abruptly dismissed from leading the transition team on November 11 largely because he had warned transition officials against hiring Flynn as national security adviser.

Two people familiar with the transition team's deliberations told Politico last month that Christie had expressed concern about Flynn being "in a leadership position."

"We were active in the effort to stymie his advances," one former transition official told the publication. "But Trump liked him. It seemed to me that they were going to take care of him."

In the days after the election, Flynn apparently crashed a meeting at Trump Tower in which Christie presented his suggestions for top national security roles in the incoming administration, Politico reported, citing people familiar with the transition. Flynn told the group — which included Ivanka Trump, Steve Bannon, and Jeff Sessions — that he wanted to be secretary of state, secretary of defense, or national security adviser, the people told Politico.

He was given the national security role in large part, reports suggest, because of his loyalty to Trump during the campaign.

"Suffice to say, I had serious misgivings, which I think have been confirmed by the fact that he pled guilty to a felony in federal court," Christie said at a press conference last week.

Asked whom he would have appointed instead of Flynn, Christie replied, "It's in about four volumes of books that were apparently thrown out the day I was terminated."

Flynn and Bannon, the former White House chief strategist who was then a top transition adviser, "celebrated" Christie's dismissal "by tossing binders full of potential personnel picks, carefully culled by Christie's team, into trash bins with a sense of ceremonial glee," according to Politico.

Christie said last week that he thought that "what folks who were involved in that transition have now painfully learned at the expense of the country is that experience matters."

Sally Yates' warnings

Sally Yates. AP

Yates, the former acting attorney general, told a Senate Judiciary subcommittee earlier this year that she had had "two in-person meetings and one phone call" with McGahn in January about Flynn's contact with Kislyak.

Yates said she called McGahn with "a very sensitive matter" she needed to discuss with him in person. She and another career Justice Department official then traveled to the White House to meet with McGahn and one of his associates in his office, where she told them that there had been news reports related to Flynn's conduct that the DOJ "knew to be untrue," she said.

Yates and the official told them "how we had this information, how we had acquired it, and how we knew that it was untrue," she recalled.

While he was vice president-elect, Pence insisted in an interview with CBS that Flynn and Kislyak "did not discuss anything having to do with the United States' decision to expel diplomats or impose censure against Russia" — a statement that turned out to be untrue and that set off alarm bells at the Justice Department.

Yates said that "the Russians also knew that General Flynn had misled the vice president and others."

"This was a problem because not only did we believe that the Russians knew this, but that they likely had proof of this information, and that created a compromise situation — a situation where the national security adviser essentially could be blackmailed by the Russians," she said. "We told them that we were giving them all of this information so that they could take action, the action that they deemed appropriate."

She continued: "I remember that Mr. McGahn asked me whether or not General Flynn should be fired, and I told him that that really wasn't our call — that was up to them."

Yates met with McGahn again on January 27, when McGahn then asked her why the DOJ would be concerned "if one White House official is lying to another," she said. He also wanted to know whether the department was pursuing a criminal case against Flynn and expressed concern that taking action against Flynn could "interfere with the FBI investigation," she said.

Yates said McGahn also asked her to see the DOJ's evidence of Flynn's conversations with Kislyak.

The FBI interviewed Flynn about his conversations with Kislyak on January 24 as part of its investigation into Russia's interference in the 2016 election. Flynn has acknowledged that he misled the federal agents by telling them that the issue of US sanctions on Russia had not been discussed when, in fact, it had.

Yates said in her testimony that she told McGahn she could not disclose how Flynn did in that FBI interview because the investigation was ongoing. But she indicated to him that Flynn told the bureau the same thing he had told Pence.

Yates called McGahn again on January 30 — hours before Trump fired her for refusing to enforce his first travel ban — to tell him he could come over to the DOJ to review the details of Flynn's communication with Kislyak.

Neither McGahn nor his lawyer has returned requests for comment about whether he ever took the DOJ up on that offer. Yates testified that, because of her firing, she didn't know whether he did.

Yates told Democratic Sen. Chris Coons that in the course of the meetings, "Mr. McGahn certainly demonstrated that he understood that this was serious." But she said she didn't know whether the White House took any additional steps to restrict Flynn's access to sensitive or classified information.

"If nothing was done," she said, "then certainly that would be concerning."